A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
To position an object, for instance the substrate table, it is common to use a so-called stage system. One type of stage system that is currently under development is a single-stage system comprising a stator extending substantially parallel to a first direction X and a second direction Y, wherein the second direction Y is perpendicular to the first direction X, and a first stage that is moveable relative to the stator in the first and second direction. A schematic example of such a single-stage system is shown in FIG. 2. The stator is indicated by the reference numeral 1 and the first stage is indicated by reference numeral 3.
The first stage 3 is provided with a system of magnets. For simplicity reasons it can be assumed that in the example of FIG. 2 the entire bottom area of the first stage is occupied by the system of magnets. The system of magnets generates a magnetic field that extends from the system of magnets to the stator underneath and in the vicinity of the first stage.
The stator is provided with an array of electric coils 5 of which only a few are indicated by reference numeral 5, the electric coils being configured to interact with the magnetic field generated by the system of magnets of the first stage in order to generate forces on the first stage to position them relative to the stator in the first and second direction.
It is noted here that the stator is usually mounted to or carried by a frame and thus acts as the stationary world. The first stage is able to move relative to the stator. The stage system is thus of the moving magnet type instead of the more commonly used moving coil type.
When basic control is used to position the stage relative to the stator, all coils on the stator are activated. However, with this configuration, most coils are not in the vicinity of the first stage with its system of magnets and thus have minimal interaction with the generated magnetic field. Furthermore, this does not allow for a second stage which can independently be positioned relative to the stator using the same coils.
To avoid this, only a subset of coils is activated, so that only the coils that have a non-negligible interaction with the magnetic field are activated, where non-negligible can be determined by the required position accuracy of the first stage. An example of a subset of coils is indicated in FIG. 2 by the bold coils 5. As shown in this example, only the coils directly under the first stage, i.e. the system of magnets, and in its direct vicinity are activated. The coils directly under the first stage are indicated by shading.
In FIG. 3, a multi-stage system is shown in which two stages, namely a first stage 3 and a second stage 7, are moveable relative to a stator 1 with multiple electric coils 5. To position the first stage 3, a first subset of electric coils is selected and activated as indicated by the bold coils. At the same time, the second stage can be positioned by selecting and activating a second subset of coils as indicated by the dashed coils.
A benefit of this configuration is that two stages can independently be positioned at the same time with respect to the same stator. However, the two stages can not approach each other closely, which makes certain types of operations impossible to perform.